I'll make a bold statement. Yui will be bigger than Sashihara.
I dig Sasshi, but she can be a bit 'out there' and sorta extreme. I'm not that familiar with Yui but it seems like she's much more focused and subtle. If she can build on her character a bit more and get a bit more experience on AKBingo and Shukan, etc, I think she can become a star. She's not there yet... (see what I did there, lol), but she's gonna be great.
She has an amazing face too. So much class and beauty, yet powerful.
In case you haven't noticed, I'm totally on the Yui bandwagon now